1. Field of the Invention
The present invention relates to the transfer of workpieces such as semiconductor wafers from a storage and transport pod to a process tool, and in particular to a system for ensuring proper registration of the pod door against the port door on the load port assembly without the use of guide pins on the port door.
2. Description of Related Art
A SMIF system proposed by the Hewlett-Packard Company is disclosed in U.S. Pat. Nos. 4,532,970 and 4,534,389. The purpose of a SMIF system is to reduce particle fluxes onto semiconductor wafers during storage and transport of the wafers through the semiconductor fabrication process. This purpose is accomplished, in part, by mechanically ensuring that during storage and transport, the gaseous media (such as air or nitrogen) surrounding the wafers is essentially stationary relative to the wafers, and by ensuring that particles from the ambient environment do not enter the immediate wafer environment.
A SMIF system has three main components: (1) minimum volume, scaled pods used for storing and transporting wafers and/or wafer cassettes; (2) an input/output (I/O) minienvironment located on a semiconductor processing tool to provide a miniature clean space (upon being filled with clean air) in which exposed wafers and/or wafer cassettes may be transferred to and from the interior of the processing tool; and (3) an interface for transferring the wafers and/or wafer cassettes between the SMIF pods and the SMIF minienvironment without exposure of the wafers or cassettes to particulates. Further details of one proposed SMIF system are described in the paper entitled xe2x80x9cSMIF: A TECHNOLOGY FOR WAFER CASSETTE TRANSFER IN VLSI MANUFACTURING,xe2x80x9d by Mihir Parikh and Ulrich Kaempf, Solid State Technology, July 1984, pp. 111-115.
Systems of the above type are concerned with particle sizes which range from below 0.02 microns (xcexcm) to above 200 xcexcm. Particles with these sizes can be very damaging in semiconductor processing because of the small geometries employed in fabricating semiconductor devices. Typical advanced semiconductor processes today employ geometries which are one-half xcexcm and under. Unwanted contamination particles which have geometries measuring greater than 0.1 xcexcm substantially interfere with 1 xcexcm geometry semiconductor devices. The trend, of course, is to have smaller and smaller semiconductor processing geometries which today in research and development labs approach 0.1 xcexcm and below. In the future, geometries will become smaller and smaller and hence smaller and smaller contamination particles and molecular contaminants become of interest.
SMIF pods are in general comprised of a pod door which mates with a pod shell to provide a sealed environment in which wafers may be stored and transferred. So called xe2x80x9cbottom openingxe2x80x9d pods are known, where the pod door is horizontally provided at the bottom of the pod, and the wafers arc supported in a cassette which is in turn supported on the pod door. It is also known to provide front opening pods referred to as front opening unified pods, or FOUPs, in which the pod door is located in a vertical plane, and the wafers are supported either in a cassette mounted within the pod shell, or to shelves mounted in the pod shell.
In order to transfer wafers between a FOUP and a process tool within a wafer fab, the FOUP is typically loaded either manually or automatedly onto a pod advance plate, which then advances the FOUP toward the process tool. The process tool includes an access port which, in the absence of a pod, is covered by a port door. Upon advance of the FOUP toward the process tool, the pod door aligns against the port door.
Once the pod is positioned on the load port, mechanisms within the port door unlatch the pod door from the pod shell and move the pod door and port door together into the process tool where the doors are then stowed away from the wafer transfer path. The pod shell remains in proximity to the interface port so as to maintain a clean environment including the interior of the process tool and the pod shell around the wafers. A wafer handling robot within the process tool may thereafter access particular wafers supported in the pod for transfer between the pod and the process tool.
The load port includes several mechanisms for ensuring that the pod and port doors properly align along the X (horizontal) and Z (vertical) axes so as to properly mate with each other. For example, the pod advance plate includes three kinematic pins that mate within respective slots on the bottom of the FOUP to ensure a precise and repeatable seating of the pod on the load port. Additionally, the port door includes a pair of latch keys, which latch keys fit within slots within the front surface of the pod door. It is the latch keys which rotate to unlatch the pod door from the pod shell and at the same time latch the pod door to the port door. The port door further includes a pair of guide pins which mate within holes in the front surface of the pod door. It is further known to provide a vacuum source around the guide pins to hold the pod door against the port door. A prior art system including the above features is disclosed in U.S. Pat. No. 5,772,386, entitled xe2x80x9cLoading and Unloading Station for Semiconductor Processing Installationsxe2x80x9d, which patent is assigned to Jenoptik A. G. Such a system is shown in FIG. 1. As seen therein, there is a pair of latch keys 10, a pair of guide pins 12 and a pair of suction cups 14 surrounding the guide pins, each for positioning the pod on the load port assembly.
The above-described mechanisms for positioning the pod with respect to the port door assembly amount to an over-constraint of the FOUP on the load port. This over-constraint has at least two significant drawbacks. First, by providing additional alignment or guiding mechanisms, this provides additional sources for particulates when the pod door is transferred to and from the port door. As indicated above, the geometries of the devices formed on semiconductor wafers are so small today that particulates of even extremely small sizes can adversely effect or ruin a device geometry on the wafer. A second drawback of over-constraint results from the fact that FOUPs occasionally deform or are improperly constructed, and tilt slightly (i.e., 10 to 30 mils) to the side while supported on the kinematic pins. When the FOUP door is aligned over the guide pins, the pins force the pod into a straightened position on the load port. This can generate particulates. Additionally, when the door is removed, the pod shell may return to its unrestrained tilted position. Thus, the FOUP door may not properly align with the FOUP shell upon return of the door to the shell. This misalignment may result in the generation of particulates, or may prevent the door from being properly returned to the shell entirely. It may also happen that FOUP sits properly on the kinematic pins, but the pod door is misaligned within the pod shell. In this event, when the pod door seats over the guide pins, particulates can be generated.
It is therefore an advantage of the present invention to provide a load port assembly that does not include excessive constraints for positioning a FOUP on the load port assembly.
It is another advantage of the present invention to minimize the amount of particulates and contaminants that may otherwise be generated as a result of contact between the pod door and guide pins.
It is a further advantage of the present invention to allow the pod door to be returned to the pod in substantially the same position from which is was acquired.
These and other advantages are provided by the present invention, which in preferred embodiments relates to a front opening interface mechanical standard, or xe2x80x9cFIMSxe2x80x9d system for ensuring proper registration of a pod door against a port door on a load port assembly without the use of guide pins on the port door. In a preferred embodiment, the load port assembly includes registration features such as kinematic pins provided to mate within slots on the bottom of a FOUP to provide a fixed and repeatable position of the FOUP on the load port assembly. The load port assembly further includes a pair of latch keys protruding outwardly from the outer surface of the port door. The latch keys are provided to mate within slots of a door latch assembly within the pod door. Once the latch keys are properly seated within the slots, the latch keys are rotated by mechanisms within the port door to decouple the pod door from the pod shell. Such rotation at the same time couples the pod door to the port door. The load port assembly may further include vacuum seals on the port door, or magnetic assemblies, to further facilitate support of the pod door on the port door. The vacuum seals or magnetic assemblies are preferably activated prior to unlocking the pod door from the shell.
With the provision of the above pod constraints on the port door, the guide pins conventionally provided on the port door may be omitted. Omission of the guide pins removes the possibility that particulates and contaminants will be generated as a result of contact between the pod door and guide pins. Additionally, without the guide pins to alter the position of the pod, there is no interference between the pod door and shell upon return of the door to the shell. Moreover, the pod door is returned to the same relative position within the pod shell from which it was acquired.
In one embodiment, the load port assembly may initially include removable alignment pins. The alignment pins operate in conjunction with a calibration fixture to set the respective and collective heights of the kinematic pins on the load port assembly. In particular, the calibration fixture includes a horizontal plate seated on the kinematic pins and a vertical plate including holes that fit over the removable pins in the port door. Once the kinematic pins are adjusted so that the vertical plate is parallel to the port door and the openings in the vertical plate mate cleanly over the removable alignment pins, the calibration fixture may be removed and the alignment pins may be removed from or retracted back into the port door. Thereafter, a FOUP may be placed on the pod advance plate and advanced to the port door.